In order to provide sound insulation in vehicles, frequently sound-absorbing sandwich panels are used that essentially comprise at least one core layer and at least one or several cover layers to cover the core layer. In this arrangement at least one of the cover layers is air-permeable and comprises a certain flow resistance. In order to reflect noise a further cover layer can also be designed so as to be air-impermeable. With a corresponding design of the core layer and of the cover layers, which, for example, comprises specifying air-permeability or porosity, air change flows from the outside into the core layer, and from the core layer to the outside, can be achieved. Among other things the sound-absorbing effect occurs in that during flowing through the porous material of a cover layer, part of the sound energy is converted to heat. In order to achieve a precisely defined sound absorbing effect, and in particular matching the expected frequency spectrum of the noise source, matching the flow resistance of at least one cover layer and of the core material is necessary. For example, DE 37 20 371 A1, DE 10 2006 023 294 A1 and WO 2007/135085 A1 show sandwich panels, each comprising at least one core layer and at least one porous or perforated cover layer.
In the state of the art it is common to manufacture such sound-absorbing sandwich panels in such a way that a core layer is provided, to which adhesive is then applied, wherein subsequently at least one cover layer is pressed onto the area of the core layer, which area comprises adhesive. After the adhesive has cured, the cover layer and the core layer are firmly interconnected to form a sandwich panel. However, there is a problem when bonding the cover layer in that, when the cover layer is pressed on, adhesive collects even in those locations of the cover layer, which locations are not directly connected to the core layer. For example, the core layer could comprise a honeycomb structure, wherein the individual honeycombs are separated from each other by honeycomb walls (hereinafter also referred to as “webs”). Ideally the cover layer is connected to adhesive only directly over the webs, however, it should be noted that from these webs the adhesive spreads to adjacent regions of the cover layer, thus at least partly blocking the cover layer that is porous or perforated. This makes it more difficult to match the fluidic characteristics of the manufactured sandwich panel to the expected frequency spectrum of the sound source. In a sandwich panel manufactured in this way it is impossible to precisely predict the percentage of the cover layer that is blocked by adhesive, and the flow resistance which the cover layer thus comprises.